Television cameras generate a video signal of an image. The video signal consists of individual lines. Each line, in turn, contains a sequence of intensity amplitudes of elements, or pixels, representing the intensity of the image at the location corresponding to each pixel.
Various standards exist as to how many lines are allocated for the creation of a sequence of television images. For example, in the United States, each video frame comprised in a television image contains two interlaced sets of 2621/2 lines, each set of 2621/2 lines is called a field. This current operating standard for television transmission in the United States is defined by the National Television Systems Committee (NTSC). This standard was approved for monochrome television transmission by the Federal Communications Commission in 1941 and further expanded for NTSC color television systems in 1954.
In this NTSC standard, the two sets of 2621/2 lines (or fields) are interlaced, i.e. their respective 2621/2 lines are interleaved so that each successive field occupies a vertical position half the vertical spacing between lines above or below the previous field. This means that the nth line in a field occupies a spacial position 1/2 line spacing above the (n+262)th line.
A video camera scanning an image needs a sample, or integration, time, for the acquisition of each pixel describing that image. The time to create a full frame of the image (consisting of two fields) is 1/30 second, each field of 2621/2 lines being generated every 1/60 second. Because of this integration time, moving pictures or objects whose position change within the 1/30 second time interval are blurred as if the image of a fast moving object were acquired with a still photography camera having a long exposure time.
Given the NTSC standard of image transmission as an example, the same area in a television image will be acquired every 1/60 second (every field) by the camera. Therefore, an object that has moved within 1/60 second will occupy different (vertical) positions in the sequence of fields. The object will therefore appear softer than the surrounding stationary areas that have not moved in the picture for multiple fields. The "spreading" of moving edges across horizontal or vertical pixels as compared to stationary objects will reduce its definition in a sequence of fields.
As described, when objects move within a sequence of video fields, they lose resolution or definition. This is objectionable in NTSC as well as in high definition television (HDTv) and other video images. A loss of image definition will be perceived greater whenever objects moving across the screen are being visually followed on the screen by the viewer. The loss of resolution or definition of the moving objects is due to the integration of the "high frequency" over an area of the scene and is referred to as "high-frequency roll-off".
One known method of reducing this loss of resolution is to reduce the integration time, i.e. reduce exposure time to acquire the image by the camera. This can be accomplished by using a shutter to reduce the exposure time as is done with still photography. Among other things, this is undesirable because it requires an increased amount of light necessary to operate the video camera thereby reducing its sensitivity.
It is therefore an object of the invention to provide a method and apparatus to equalize for the loss of resolution in a reproduced television signal caused by a movement of an object during the integration time of the video camera.
It is another object of the invention to provide a method and apparatus for creating a motion aperture signal that can be added to a video signal to enhance the high frequency loss of the video signal due to the effects of motion.
It is still another object of the invention to provide a non-adaptive method and apparatus for motion aperture correction utilizing a linear filter to enhance high frequency motion resolution of an interlaced video image.